The lithium-ion battery cell is the premier high-energy rechargeable energy storage technology of the present day. Unfortunately, its high performance still falls short of energy density goals in applications ranging from telecommunications to biomedical. Although a number of factors within the cell contribute to this performance parameter, the most crucial ones relate to how much energy can be stored in the electrode materials of the cell. Based on electrodes utilizing intercalation processes, the present day state of the art Li-ion battery technology exhibits an energy density in excess of 200 Wh/kg and 420 Wh/l. The energy density of lithium battery technology is far less than half of the theoretical energy densities that could be achieved. The technology is currently limited by the energy density of the positive electrode. This is due to intercalation reactions limiting the amount of Li+ inserted, thereby limiting electron transfer to typically less than 1e− per compound such as LiMeO2, where Me is a transition metal.
During the course of development of rechargeable electrochemical cells, such as lithium (Li) and lithium-ion battery cells and the like, numerous materials capable of reversibly accommodating lithium ions have been investigated. Materials of the present invention include conversion and reverse conversion compounds which allow for multiple electrons to be transferred to the active electrode to reduce fully to the metal state plus lithium salt and then subsequently reoxidize back to the original compounds. Existing state of the art materials include occlusion and intercalation materials, such as carbonaceous compounds, layered transition metal oxide, and three dimensional pathway spinels which have proven to be particularly well-suited to such applications. However, even while performing reasonably well in recycling electrical storage systems of significant capacity, many of these materials exhibit detrimental properties, such as marginal environmental compatibility and safety, which detract from the ultimate acceptability of the rechargeable cells. In addition, some of the more promising materials are available only at costs that limit widespread use. However, of most importance is the fact that the present state of the art materials have the capability to store relatively low capacity of charge per weight of material (specific capacity, mAh/g) or energy per weight (specific energy, Wh/kg).
Materials of choice in the fabrication of rechargeable battery cells, particularly highly desirable and broadly implemented Li-ion cells, have for some considerable time centered upon graphitic negative electrode compositions, which provide respectable capacity levels in the range of 300 mAh/g. Complementary positive electrode materials in present cells use less effective layered intercalation compounds, such as LiCoO2, which generally provides capacities in the range of 150 mAh/g. Alternative intercalation materials, such as LiNiO2, and LiMn2O4, have more recently gained favor in the industry, since, although exhibiting no appreciable increase in specific capacity, these compounds are available at lower cost and provide a greater margin of environmental acceptability.
Due to increasing demand for ever more compact electrical energy storage and delivery systems for all manner of advancing technologies, the search continues for battery cell materials capable of, on the one hand, providing greater specific capacity over wider ranges of cycling rates, voltages, and operating temperatures, while, on the other hand, presenting fewer environmental hazards and greater availability at lower processing and fabrication costs.
In the search of material systems which can deliver much higher specific capacities and energy, interest has shifted to examination of the more active fluoride compounds. Primary metal fluorides have been known for well over 30 years as attractive electrode materials, however, the higher voltage materials exhibit a high bandgap resulting in insulator properties and very poor electrochemical activity. Recently, reversible conversion reactions in metal fluorides have been shown to occur. Badway et al. (Journal of The Electrochemical Society, 150(9) A1209-A1218 (2003)) reported the use of carbon metal fluoride nanocomposites to enable the electrochemical activity of metal fluorides. Their studies have shown that reducing the particle size of high bandgap, insulating metal fluorides to the nanodimensions in combination with highly conductive carbon resulted in the enablement of a new metal fluoride conversion process resulting in a major improvement in specific capacity relative to current state of the art. Badway et al. reported greater than 90% recovery of the FeF3 theoretical capacity (less than 600 mAh/g) in the 4.5-1.5 V region through reversible conversion, which is a fundamentally different energy storage mechanism compared with the present state of the art intercalation.
Until recently, full utilization of certain metal fluorides, such as copper fluoride, has not been realized. Researchers have tried to enable this high energy density compound for more than 30 years with only limited success because of poor utilization of the material. Copper fluoride has a theoretical conversion potential of approximately 3.2 V and a discharge specific capacity of approximately 520 mAh/g. This leads to an exceptionally high energy density in excess of 1500 Wh/kg. Such capacity values are over 300% higher than those attained in present day state-of-the-art rechargeable Li battery cells based on LiCoO2 intercalation compounds. With respect to existing primary cathode compounds, copper fluoride would exceed the widely utilized MnO2 energy density by almost a factor of two and copper fluoride compounds exceed the volumetric energy density of copper monofluoride by 20-30%. U.S. Patent Publication No. 2006/0019163, which is hereby incorporated by reference discusses nanocomposite technology that has enabled greater than 99% of the theoretical specific capacity of copper fluoride.
Despite the promising energy densities extracted for a number of the metal fluoride nanocomposite technologies, challenges still exist. For the reversible conversion metal fluorides, the ability to retain a larger percentage of the capacity during reversible cycling is desired. In copper fluoride nanocomposites, the material has a poor ability to retain charge when stored at elevated temperatures in its partially discharged state. For example, state of the art materials typically lose 100% of their capacity after one week at 40° C. or 60° C. when they are previously partially discharged. These present a great challenge to researchers where no obvious methodology exists to improve such performances.
Hence, there is a need in the art for electrical energy-storage and delivery systems that utilize copper fluoride effectively.